It is the trend of the energy development to balance the energy, the environment and the economics. Furthermore, there are many drawbacks of the traditional centralized power generation technology. In comparison with the centralized power generation technology, the advantages of the distributed power generation technology are the flexible power generation, the environment protection, etc. The distributed power generation technology not only meets the requirements of the power system and the user, but also provides the flexibility, the reliability and the economical value.
Currently, the solar power generation system, the fuel battery and the wind power generation system are the main distributed power generation systems. The distributed power generation technology can be independently operated or be operated with the power generation system. The distributed power generation technology is operated in the place without the power grid. The distributed power generation technology is operated with the power generation system in the area with the increased loading. The ac generating circuit is the important part of the distributed power generation system.
Please refer to FIG. 1(a) showing the three-level ac generating circuit according to the prior art. Since the three-level ac generating circuit 10 is a single-stage circuit, the three-level ac generating circuit 10 has higher efficiency but needs higher battery voltage.
Please refer to FIG. 1(b) showing another three-level ac generating circuit according to the prior art. The three-level ac generating circuit 11 is a two-stage circuit. The battery voltage is converted into a predetermined value via the pre-converter 111, and then output via the post-inverter 112. Typically, the typical pre-converter 111 is a boost circuit or a buck-boost circuit.
The circuit shown in FIG. 1(b) has the expanded variation of the input voltage, but the drawbacks thereof are those the efficiency is low and many interferences are formed from the capacitor. In addition, the traditional half-bridge structure is used in the post-inverter 112, and the efficiency is low while the high voltage is applied to the post-inverter 112. The traditional half-bridge switch component has the anti-parallel diode as the continuous loop, so that while the switch component is conducted, the reverse recovery is performed by the anti-parallel diode. The MOSFET is not used as the switch component for minimizing the loss from the reverse recovery since the parasitic diode property of the MOSFET is not good enough even though the MOSFET has the conduction property of the resistance and good switch property. The switch loss of the system cannot be minimized by using the MOSFET as the switch component.
Generally, the three-level inverting circuit is used for the device with the high direct current input voltage but not the device with the low voltage. The voltage endurance capability of the switch component is decreased by the three-level inverting circuit, so that the efficiency of the device with the high voltage having the three-level inverting circuit is increased; therefore, the three-level inverting circuit is generally used in the fields of the uninterrupted power system and the motor driving. In the fields of the uninterrupted power system and the motor driving, the output terminal of the three-level inverting circuit is connected to the power-consuming device, and the load characteristic of the power-consuming device is changed along with the operation status of the power-consuming device. It means that the output voltage and the current phase of the three-level inverting circuit are changeable, i.e. there may be dual directions of the energy transportation.
Please refer to FIG. 2(a), which is the oscillogram showing the driving signals for driving the four switches by the single phase three-level inverting circuit with the inductance load. The switch components Sx1 and Sx2 are main control components, and the switch components Sx2 and Sx3 have the driving signals complementary to the diving signals of the switch components Sx4 and Sx1, respectively. FIGS. 2(b)-2(e) are circuits showing the three-level inverting circuit operating at different statuses.
FIG. 2(b) and FIG. 2(d) are circuits showing the three-level inverting circuit in the modes of the energy transportation. In FIG. 2(b), the switch component Sx1 is switched at high frequency and has the driving signal complementary to the driving signal of the switch component Sx3, the switch component Sx2 is normal on, and the switch component Sx4 is normal off. While the switch component Sx1 is conducted, the energy is transported from the input terminal via the switch components Sx1 and Sx2 and the inductor Lx to the output terminal. While the switch component is off, the inductor current iLx passes through the diode Dx12 and the switch component Sx2.
The operation procedure shown on FIG. 2(d) is similar to that shown on FIG. 2(b).
FIG. 2(c) and FIG. 2(e) are circuits showing the three-level inverting circuit in the mode of the energy feedback. In FIG. 2(c), the switch component Sx1 is switched at high frequency and has the driving signal complementary to the driving signal of the switch component Sx3, the switch component Sx2 is normal on, and the switch component Sx4 is normal off. While the switch component Sx1 is conducted, the inductor current iLx is negative, so that the current passes through the diode anti-parallel to the switch components Sx1 and Sx2, and the energy is feedback from the output terminal to the input terminal. While the switch component Sx1 is off and the switch component Sx3 is conducted, the inductor current iLx passes through the switch component Sx3 and the diode Dx34.
The operation procedure shown on FIG. 2(e) is similar to that shown on FIG. 2(c).
According to the above analyses, while the three-level inverting circuit is in the mode of the energy transportation (for example, as shown in FIG. 2(b)), the switch component Sx3 is not the component for passing through the current, so that the switch component Sx3 is switched at low frequency, i.e. the switch frequency is equal to the frequency of output voltage. While the three-level inverting circuit is in the mode of energy feedback, the switch component Sx3 is as the component for passing through the current, so that the switch component Sx3 has to be switched at high frequency. In addition, there are the energy transportation mode and the energy feedback mode in the fields of the uninterrupted power system and the motor driving, i.e. there are dual directions of the energy transportation, so that the switch component Sx3 has to be switched at high frequency. Similarly, the switch component Sx2 also has to be switched at high frequency.
In addition, while the three-level inverting circuit is in the mode of the energy feedback (for example, as shown FIG. 2(c)), the current passes through the diode anti-parallel to the switch components Sx1 and Sx2, so that while the switch component Sx1 is off and the switch component Sx3 is conducted, the diode anti-parallel to the switch components Sx1 and Sx2 is reversely recovered. There is the parasitic diode in the MOSFET, and the reverse recovery characteristic of the parasitic diode is very bad, so that while the three-level inverting circuit is used in the fields of the uninterrupted power system and the motor driving, the MOSFET is generally not used as the switch component. The MOSFET is not used as the switch component for minimizing the loss from the reverse recovery since the parasitic diode property of the MOSFET is not good enough even though the MOSFET has the conduction property of the resistance and good switch property.
In addition, since the power systems, such as the solar battery and the fuel battery, have the wide range of the voltage variation of the direct current power source, wherein the highest input voltage may be several times (for example, more than 3 times)of the lowest input voltage, the three-level inverting circuit cannot be directly used in the power system.
In order to overcome the disadvantages of the prior art described above, the present invention provides a three-level AC generating circuit and the control method thereof.